1. Field of the Invention
This invention relates to a system for digitally storing high frequency signals and more particularly to a system for storing high frequency signals with the ability to retrieve and transmit stored signals repetitively or with various selectable delays. This type of system can be useful in the signal processing field as applied to radio, TV, satellite transmission, secure transmission, radar and communication jammers, etc.
2. Description of Related Art
One method of storing wideband high frequency signals is to down-convert the high frequency pulse signal to an intermediate frequency by local oscillation mixing for sampling and digitization. The digital words generated by analog-to-digital conversion are stored in a digital memory. The output signal is reconstructed by performing a digital-to-analog conversion on the stored contents of the digital memory, reconstituting the I.F. frequency. This frequency is then up-converted reproducing the original signal. This system has been previously described in ELECTRONIC WARFARE, "A Coherent Microwave Memory Using Digital Storage": "The Loopless Memory Loop", by S. C. Spector, January/February 1975, pages 108-110.
An extension of the above technique is to mix the incoming signal with a number of local oscillators having among them a common frequency but having different phases. The result of the mixing process is two sets of frequencies for each mixing signal, one set representing the sum frequencies and the other set representing the different frequencies. The sum phase frequencies are eliminated by filtering and the phase difference signals are quantized using digital techniques. The process of reconstructing the original signal involves mixing the stored digital signal after digital-to-analog conversion by signals having the frequency and phase of the original local oscillation mixing signals. The resulting product signals are summed and result in the reconstruction of the original signal. This system is further described in U.S. Pat. No. 3,947,827 which issued on Mar. 30, 1976 naming J. L. Dantremont, Jr., et al, as inventors.
An analog method of storing the signal is to introduce the high frequency signal into a memory loop. The received signal is amplified by an input travelling wave tube (TWT) and the loop TWT (for in-line memory operation) where it is divided into two paths. One path is the time-delayed section of the circulation loop and the other is coupled directly to the output TWT. After a time delay of T, the switch opens and the input path closes the loop just as the high frequency signal appears at the output to the delay line. The high frequency signal thus will continue to circulate, reproducing the input frequency.
Another method of analog storage is a parallel sampling approach described in applicant's Canadian Pat. No. 1,135,342 which issued on Nov. 9, 1982 naming T. W. Tucker, L. J. Conway, and S. L. Bouchard as inventors. This patent describes how the high frequency signal waveform is distributed along the delay line and is sampled at a number of points. The sampled voltages are then stored in analog memories. The signal is reconstructed by the reverse process of applying the stored voltage samples to an output delay line which is similar to the input line.
One problem with the first reference digital approach described above is the upper frequency limitation imposed by the speed of the current digitization circuitry which is limited to several hundred megahertz. Even the multi-phase approach has an instantaneous bandwidth limitation insofar as increased bandwidth can only be accommodated by additional mixer stages. The practical limitation in the number of stages would probably allow bandwidths to 1 GHz or so.
A second problem is the frequency and phase tracking requirements of the mixers. Any deviation of the frequency and/or phase between the input mixer and the output mixer will alter the quality of reconstruction of the stored signal.
Problems with the recirculating memory-loop as described are that phase coherence is retained only if the return delay is greater than the width of the input pulse signal. If the input pulse is longer than the return delay path the leading edge of the pulse will overlap the trailing edge of the pulse giving rise to constructive or destructive interference which destroys phase coherence. A second problem is that the complete pulse can only be recalled at increments in time which correspond to the recirculation time of the loop. Equally both analog approaches have limited storage times and therefore cannot be recalled at times greater than a few microseconds.